The loss suffered by qubits during direct transmission over waveguide channels remains a challenge that limits the throughput of quantum communications. Contrary to the classical case, in the analogous quantum case, an unknown qubit cannot be measured and regenerated without adding errors as a consequence of the no cloning theorem.
Quantum repeater networks have been proposed, using an “offline” step to distribute, repair, and store entangled qubit pairs before they are needed. In such a case, when a qubit needs to be transported, the pre-distributed entanglement can be used to connect two locations with a shared entangled pair, followed by teleportation of the qubit. Thus, the “runtime” quantum cost is not directly related to the channel loss. However, even if fully realized, complexity and cost of quantum repeater networks may not be required for all potential quantum communications applications.